3. Results
3.1 Purification and characterization of synaptosomes
Figure 3‐1: Experimental outline of the project.
The schematic illustrates the experimental flow performed to determine the pre‐synaptic architecture: synaptosomes were purified and characterized (3.1). Next, their physical characteristics were determined using 3D reconstruction EM (3.2) followed by absolute protein quantification per synapse (3.3) and determining the organization of the proteins within the synapse (3.4). The information obtained from these sets of experiments was then used to generate a graphical model of the average pre‐synaptic terminal (3.4).
3.1 Purification and characterization of synaptosomes
As outlined in the previous section, the main goal of this study was to investigate the molecular architecture of a pre‐synaptic terminal. The model system I used to address this question was isolated nerve terminals – so called synaptosomes ‐ from rat brains. During homogenization of the brain material, the pre‐synaptic terminals get ripped off of the neuronal processes. These are then isolated via consecutive differential and gradient centrifugation steps (see detailed protocol in 2.5). In principle, the synaptosomes are intact and maintain their neurophysiological properties (Fischer von Mollard et al., 1991). For this study I used only cortex and cerebellum as starting brain material. By using only these two areas of the brain I was able to limit the variability of synapse populations to a certain extend and ensured that I still had enough brain material per animal to perform multiple quantification experiments.
For the purpose of investigating the molecular architecture of the terminal, it was not crucial to work with ultra‐pure synaptosome preparation (Dunkley et al., 2008). As outlined in the next sections, the contaminations I found in the synaptosome preparations were mainly of mitochondrial and myelin origin. These types of contamination neither biased the determination of the physical characteristics (3.2) nor the protein quantification (3.3, as all proteins investigated are majorly pre‐synaptic). However, especially the latter depended entirely on actually knowing how pure the fractions were – i.e. how many synaptosomes are present per µg of total protein. To address this question I developed two different assays. The first relied on determining the amount of synaptosome particles in a fluorescence‐based assay and the second on identifying the different components per fraction in an ultrastructural assay. The results of these two approaches are outlined in the next paragraphs (for detailed protocols refer to section 2.7) while a comparison of the two assays as well as an evaluation of the results is provided in 3.1.3.
3.1.1 Determining the fraction of synaptosome particles using a fluorescence assay
The first assay relied on determining the amount of synaptosomes from all particles in a synaptosome fraction using fluorescence microscopy. For this purpose, defined amounts of the synaptosome fractions were immobilized on glass cover‐slips via centrifugation (spin down) and immunolabeled for different marker proteins: (i) Synaptotagmin 1 as a marker for SVs and ultimately synaptosomes, (ii) Bassoon or PSD‐95 as markers for the pre‐ or the post‐synaptic compartment respectively. As a positive control – i.e. a marker which labels all particles – I used TMA‐DPH which is a hydrophobic fluorescent probe used to label membranes (Illinger et al., 1989). This allowed me to determine the total number of particles (TMA‐DPH labeling) and the fraction of synaptosome particles (defined as Synaptotagmin 1 positive particles). Knowing the amount of starting material which was spun onto the cover‐slips as well as the imaging area, it is possible to calculate the absolute amount of synaptosomes in each preparation. Representative images from this assay can be found in Figure 3‐2 A. Analyzing the images from seven to eight independent experiments per condition showed that all synaptosome preparations (S1‐4) contained between 51 and 54 % of synaptosome particles.
Figure 3‐2: Fluorescence spin down assay to determine purity of synaptosome preparations.
(A) Representative images from the fluorescence spin down assay. The immobilized particles are immunolabeled for Bassoon (red) and Synaptotagmin (green). The total pool of particles is labeled with TMA‐DPH (blue). Size bar is 2.5 µm.
(B) Quantification of the data obtained in the fluorescence spin down assay. Bars represent the fraction of synaptosome particles in the four different preparations (S1‐4). Graph show mean ± SEM from at least 7 independent experiments.
However, as this assay is critically dependent on how efficient the particles are immobilized (spun down) on the cover‐slips it is necessary to determine the efficiency of the spin down. This was done comparing the amount of two prominent pre‐synaptic marker proteins – Syntaxin 1 and Synaptobrevin 2 – in the supernatant after spin down (sample) with the input material prior to spin down (control). The results of this control experiment are displayed in Figure 3‐3: the average efficiency of the spin down was 98.68 % (mean of 99.63 % for Syntaxin 1 and 97.74% for Synaptobrevin 2) indicating that there was hardly any synaptosomes which did not get immobilized on the cover‐slips. Nevertheless, this value was used as a correction factor for determining the amount of synaptosomes per fraction (i.e. values in Table 3‐1 are corrected for the loss during centrifugation).
Although the synaptosomes are substantially diluted – and therefore well separated – before being immobilized it is still possible that some particles might have been aggregated into clumps. In this case it would have been difficult to discern them using diffraction limited microscopy, hence aggregation of synaptosomes pose a substantial bias to the quantification of the number of synaptosomes per preparation. To test if this is the case I designed a second assay, which relies on EM to determine the absolute amount of synaptosomes. The results of this assay are outlined in the following section.
Figure 3‐3: Spin down efficiency assay.
(A) Example blot showing the abundance of two pre‐synaptic marker proteins – Syntaxin 1 (Syx 1) and Synaptobrevin 2 (Syb 2) – in the supernatant prior to (Control) and after (Sample) centrifugation. As indicated by the blots, there is hardly any protein detected in the supernatant after spin down indicating that most of the synaptosomes are immobilized on the cover‐slip.
(B) Graph shows the spin down efficiency for the two marker proteins Syntaxin 1 and Synaptobrevin 2. The dotted red line marks 100% spin down efficiency i.e. that all synaptosomes are immobilized on the cover‐slip. Graph shows mean ± SEM from 8 independent experiments.
3.1.2 Determining amount of synaptosomes using electron micrographs
In order to test the results obtained by the fluorescence spin down assay I investigated electron micrographs of the synaptosome preparations. As described in 2.7.2, all visible particles in the EM images were first manually outlined/ selected and identified. This showed that besides the synaptosomes the majority of the sample was composed of mitochondria, myelin and post‐synapses. However, a small amount of objects (~14%) could not be identified as they were either too small or unspecific in appearance.
This data could further be used to control the quantification results obtained previously from the fluorescence spin down assay. Unlike the spin down assay, this assay was not biased by an accumulation of particles into clumps as all objects could be discerned individually in EM. In order to determine the amount of synaptosomes per fraction, the relative volume of each particle group (synaptosome, post‐synapses, mitochondria, myelin and unknown) was determined (see Figure 3‐4). The fraction obtained for synaptosomes (i.e. the relative volume occupied by synaptosomes in the fraction) could then be used as a control for the results of the spin down assay.
Figure 3‐4: EM‐based assay to determine composition of synaptosome preparations.
Graph shows the relative composition of the synaptosome preparations. Results are derived by analyzing thin sectioned electron micrographs. Graph shows mean ± SEM of the four different synaptosome preparations.
3.1.3 The purity of the synaptosome preparations – comparing the two assays
As outlined in the previous two sections, two different assays were employed to determine the amount of synaptosomes per preparation. The first assay depended on fluorescence microscopy to identify the amount of synaptosome particles from all particles.
Results obtained with this assay where then validated using an EM‐based assay in which the relative volume occupied by the synaptosomes in the preparation was analyzed. In Figure 3‐5, the results of these two assays are compared with each other: the bars represent the relative amount of synaptosomes determined with the fluorescence (black) and the EM‐based (grey) assay.
In summary, both assays delivered strikingly similar results concerning the amount of synaptosomes per preparation. In addition, the EM‐based assay allowed characterization of other components of the synaptosome fractions. The results (i.e. absolute numbers of
synaptosomes) of the assays are summarized in Table 3‐2. These values are crucial for further quantitative experiments using the synaptosomes and will be used later to determine absolute protein numbers per pre‐synaptic terminal (see 3.3).
Figure 3‐5: EM‐based assay confirms findings of the fluorescence spin down assay.
Graph shows the fraction of synaptosomes per preparation (S1‐4) as determined with the fluorescence spin down assay (black bars) compared to the EM‐based assay (grey bars). For the fluorescence assay, bars represent mean ± SEM of at least 7 independent experiments. The bars of the EM assay (grey) do not have error bars as they are derived from several images of only a single experiment each (i.e. the respective preparation).
Table 3‐1: Absolute numbers of synaptosomes per nanogram of preparation.
Preparation Synaptosomes per ng
S 1 8532
S 2 10602
S 3 10518
S 4 10096